The present invention relates to techniques for placing external casing packers (ECP) outside slotted liners. In particular, the invention relates to chemical external casing packers (CECP) for such a purpose.
In traditional well completions, a casing, typically made of steel, is positioned in the well and the annulus between the casing and the well filled with cement. Fluid communication between the reservoir and the well is usually achieved by perforating the casing and the cement sheath using an explosive charge inside the casing so as to create a fluid communication path. Fluid flow along this path can be enhanced or stimulated by fracturing and/or placement of proppant or the like. However, this method of completion is not necessary the most economical for particular well types, especially horizontal producing sections. In such cases, slotted liners can be used as completion devices when formation characteristics are adequate. Slotted liners are installed without cementing leaving the annulus free for fluid communication, the liner being held in place by centralizers or the like. This completion method can allow optimized production, as flow cross-section near the well bore can be maximized.
One of the main problems of this completion is the difficulty to isolate some sections of the well during production as may be required when one section of the well produces an unwanted fluid (i.e. water). A conventional approach to prevent unwanted flow from a zone in a traditional completion would consist of installing a valve (or a bridge plug) in the well bore to stop fluid from flowing to the zone below that device. However with slotted liner, this isolation within the well bore is ineffective, as fluid can flow in around the device by the annulus outside the slotted liner.
To ensure proper isolation, it is therefore necessary to plug the annulus in the area of the valve or plug. This isolation can be achieved by external casing packer (ECP). Typically, this is a device with an external rubber membrane installed between slotted liner sections, while running the liner in the well. When required, this rubber membrane can be inflated with cement to plug the annulus. This isolation process is often inadequate and the rubber often cannot seal properly against the formation. In some case, the rubber membrane is damaged during the installation and cannot inflate properly.
Another technique for annular isolation is based on chemical injected in the annulus at the proper position as is described in U.S. Pat. No. 5,697,441. The chemical needs to have the proper properties to block the annular flow, for example having thixotropic properties to develop a yield strength to resist the shear force generated by the formation fluid in the annulus. It can also be arranged to set to become hard (such as cement). The main problem with the chemical external casing packer (CECP) is the improper filling of the annulus which can arise for different reasons, for example:
Gravity can lead to some segregation of the chemical in the annulus.
Even with the best adjustment of viscosity, it is rare that the chemical will flow in the annulus to ensure full coverage of the annulus (the fluid will tend to follow the path of least resistance) and there is no real mechanism to force the fluid to progress in a radial direction towards the formation and fill the annulus.
In practice, CECP""s often leak but they do have the advantage that they can be installed at any position in the slotted liner.
The use of magnetic cement slurries, spacers, etc. has been previously proposed in U.S. Pat. Nos. 4,691,774 and 4,802,534. Magnetic particles are incorporated in the fluids to make them susceptible to manipulation by magnetic fields. In particular, this is used to obtain a scrubbing action in the well to remove deposits remaining in the well when the cement is placed which would otherwise prevent a good cement bond from forming. The manipulation of the fluids is achieved by means of a device placed inside the casing which creates an oscillating magnetic field in the location of the magnetic fluid.
The present invention utilizes the properties of magnetic fluids to improve the performance of CECP""s.
In accordance with a first aspect of the invention, there is provided a method of sealing an annulus surrounding a slotted liner in a well, comprising: generating a magnetic field in the annulus in a region to be sealed; and injecting into the region a sealing fluid comprising magnetic particles such that the fluid is substantially confined to fill the annulus in the region to be sealed by the interaction of the magnetic particles and the magnetic field.
Preferably, the magnetic field is generated by means of magnets positioned on the outside of the liner and/or inside the liner, adjacent the region to be sealed. The magnets can be positioned on either side of the region to be sealed to confine the sealing fluid therebetween.
Magnets on the outside of the casing can comprise, for example, opposed horseshoe magnets positioned on either side of the region. Multiple rows of magnets can be used if desired.
Where magnets are positioned inside the liner, it is particularly preferred that these be moveable within the liner. In such cases, it is preferred to provide an external magnet structure, for example an apparent pole typically made from a high mu metal (a metal having a high value of magnetic permeability) or a rare-earth magnetic material (eg. Smxe2x80x94Co, Ndxe2x80x94Fe,xe2x80x94B). When the magnet is positioned inside the liner near the external magnet structure, the two together define a xe2x80x9chorseshoexe2x80x9d structure. The external magnet structure can conveniently be located inside a centralizer spring.
In accordance with a second aspect of the invention, there is provided a slotted liner for a well, comprising: injection ports for injecting a fluid including magnetic particles into the annulus surrounding the liner; and at least one magnet for generating a magnetic field around the injection ports so as to confine the fluid to fill the annulus around the injection ports.
One preferred embodiment has at least one pair of opposed rows of horseshoe magnet structures positioned on the outside of the liner. These can comprise permanent magnets, or external magnet structures which cooperate with a magnet inside the liner to generate the magnetic field.
Where an external magnet structure is used, it is preferably formed from a high-mu metal or rare earth magnet and can be conveniently located inside a bow spring centralizer for protection. The magnet inside the liner can be movable and when positioned next to the external magnet structure, the two cooperate to generate the magnetic field in the annulus.
The portion of the liner comprising the injection ports typically has no other perforations and is conveniently formed from a non-magnetic material.
In accordance with a third aspect of the invention, there is provided a method for sealing an annulus surrounding a slotted liner in a well, comprising pumping a fluid comprising magnetized particles into the annulus in the region to be sealed at a rate sufficient to allow the magnetized particles to agglomerate and substantially fill the annulus in the region to be sealed.
The pumping rate and the viscosity of the fluid are selected such that the effect of the magnetized particles is to hold the fluid in place while the pumping takes place.
It is particularly preferred that a setting fluid is used, for example a hydraulic cement.